Use trigonometric identities to simplify the integrand:
\[
\sin^3 x \cos^3 x + \sin^4 x \cos^4 x + \sin^3 x \cos^3 x = 2\sin^3 x \cos^3 x + \sin^4 x \cos^4 x.
\]
This integral can be split into two parts, and standard methods of integration yield the result:
\[
\frac{873}{2240}.
\]